Generally, CRTs are provided with an in-line electron gun where three cathodes are arranged in a horizontal line to emit thermal electrons. The thermal electrons emitted from the cathodes pass through a plurality of grid electrodes and a shield cup while being focused and accelerated to form three electron beams for exciting three different phosphors that produce the three primary colors of red (R), green (G), and blue (B).
In order to excite the correct phosphors, the electron beam should be converged on one point of the screen. For this purpose, the electron beam is deflected by a deflection yoke placed around the outer periphery of the funnel and passes through a beam-guide aperture of the color-selecting shadow mask. This convergence state becomes a critical factor for the resolution of the CRT.
It is known that the resolution of the CRT can be improved through increasing screen image constituting signals by enhancing a horizontal deflection frequency of the deflection yoke. However, with this method, the electron beam is liable to be diverged on the screen. This divergence can be explained on the basis of Lenz's law.
According to Lenz's law, when a changing magnetic field crosses a conductor, an induced electromotive force is produced across the conductor in such a direction as to oppose the change that produces it.
In this connection, the shield cup formed with conductive materials acts as the conductor. When the deflection yoke generates a strong magnetic field with the enhanced horizontal deflection frequency, the magnetic field heavily influences the shield cup. The magnetic fields are initially directed from left to right because the horizontally deflected electron beam is scanning the screen in that direction. The direction of the magnetic field is then abruptly changed from right to left to perform the scanning from the left side of the screen and, hence, an induced electromotive force is produced across the shield cup in a direction opposite to the change of the magnetic field.
Such an induced electromotive force is formed intensely in the vicinity of the G beam-guide hole of the shield cup due to the structure of the CRT. With the induced electromotive force, as shown in FIG. 6, the G beam should be deflected toward the left side S1 of the screen with a smaller size than the R and B beams. On the contrary, the G beam is deflected toward the right side S2 with a larger size than the R and B beams. This divergence, called "a horizontal center raster (HCR) phenomenon", causes the resolution at the peripheral portion of the screen to seriously deteriorate.